This invention relates to digital communication systems and methods and more specifically to transmission and reception of coded digital bit streams.
Digital bit streams are transmitted and received in many digital communication systems including but not limited to data storage systems and wireless and wired data communication systems. As the capacity and utilization of these systems continues to increase, it is generally desirable to transmit bit streams at higher data rates. Higher data rates may be obtained by operating the systems more rapidly. However, it is also desirable to increase data rates by coding more information into a bit stream.
The need for higher data rates will now be explained for wireless communications including but not limited to wireless radiotelephone communication systems and methods. Wireless communications continue to expand as more radio spectrum becomes available for commercial use and as cellular radiotelephones become commonplace. In addition, there is currently an evolution from analog communications to digital communications. Speech is represented by a series of bits, which are modulated and transmitted from a base station to a radiotelephone. The radiotelephone demodulates the received waveform to recover the bits, which are then converted back into speech. There is also a growing demand for data services, such as e-mail and Internet access, which generally use digital communications.
There are many types of wireless digital communication systems and methods. Traditionally, Frequency-Division-Multiple-Access (FDMA) is used to divide the spectrum into a plurality of radio channels corresponding to different carrier frequencies. These carrier frequencies may be further divided into time slots, referred to as Time-Division-Multiple-Access (TDMA), as is the case in the D-AMPS, PDC, and GSM digital cellular radiotelephone systems. Alternatively, multiple users can use the same bandwidth using spread spectrum techniques such as Code-Division-Multiple-Access (CDMA). IS-95 and J-STD-008 are examples of CDMA standards.
Regardless of the modulation and multiple access methods used, the system design should efficiently use the limited available radio spectrum. Thus, there continues to be a need to efficiently send as many bits of information as possible through the radio channel with a given quality.
Higher data rates may be achieved using many approaches, which may apply to FDMA, TDMA, and CDMA systems. One approach is to trade data rate for accuracy. For example, by reducing forward error correcting encoding rates or spreading factors, higher data rates may be achieved at the expense of increased bit error or frame error. Also, higher order modulation can be used.
A second approach is to trade data rate for user capacity. This approach gives a user more xe2x80x9cchannels,xe2x80x9d i.e., more FDMA carriers, more TDMA time slots, or more CDMA codes. This may increase the data rate of one user, but may leave fewer channels available for other users.
Both of these approaches may involve trade-offs. Accordingly, it is desirable to allow an increase in data rate without the need to increase the symbol transmission rate.
It is therefore an object of the present invention to provide improved digital communication systems and methods.
It is another object of the present invention to provide digital communication systems and methods that can increase the amount of information that is encoded in a bit stream without the need to increase the symbol transmission rate.
These and other objects are provided according to the present invention by combining block encoding and differential and/or amplitude modulation. Conventional block encoded systems and methods send one of N codewords to transmit m=log2 (N) bits. According to the invention, for every codeword period, one or more additional bits can be transmitted by applying differential modulation and/or amplitude modulation to the codewords. In differential modulation, additional information may be sent by flipping or not flipping the sign of a codeword. In amplitude modulation, additional information may be sent by changing the amplitude of successive codewords. A combination of differential and amplitude encoding may also be used. Thus, the data rate can be increased without increasing the symbol transmission rate.
More specifically, transmitters and transmitting methods according to the invention transmit a first bit stream and a second bit stream by block encoding the first bit stream to produce codewords. The second bit stream is differential and/or amplitude encoded by a second encoder that uses at least one of differential and amplitude encoding to produce secondary symbols. The secondary symbols are modulated onto the codewords to produce modified codewords. A carrier wave is then modulated according to the modified codewords. Thus, each codeword can include supplementary information thereon that is generated by the differential and/or amplitude encoding of the second bit stream.
The first bit stream may be block encoded using an orthogonal code, Nordstrom-Robinson code, Reed-Muller code or Kerdock code. Differential encoding may be provided by performing at least one EXCLUSIVE-OR operation on the secondary symbols and the codewords. Many forms of modulation may be used including direct sequence spreading and offset QPSK modulation.
Propagated signals according to the invention include a first bit stream and a second bit stream embodied in a carrier wave. The first bit stream and the second bit stream are embodied in a carrier wave by a plurality of codewords that are modulated on the carrier wave. The codewords comprise a block encoding of the first bit stream with differential and/or amplitude encoding of the second bit stream modulated thereon.
Receivers and receiving methods according to the present invention correlate a bit stream to a set of codewords to produce correlations. The highest correlations are detected to thereby detect codewords that correspond to the bit stream. The sign and/or amplitude changes in the correlations corresponding to the detected codewords are also detected to thereby detect differential and/or amplitude encoded secondary symbols in the bit stream.
The highest correlations may be detected using a soft decision block code detector or a hard decision block code detector. Sign and/or amplitude changes may also be detected using a soft decision differential and/or amplitude detector or a hard decision differential and/or amplitude detector. The highest correlations may be detected by squaring the magnitude of the correlations. The sign and/or amplitude changes in the correlations may be detected by delaying a correlation corresponding to a detected codeword and multiplying the delayed correlation with a current correlation to detect differentially encoded secondary symbols in the bit stream.
Transmitters and receivers as described above may be combined to provide digital communication systems. Transmitting and receiving methods as described above may be combined to provide digital communication methods. Accordingly, additional bits may be transmitted and received in each codeword period to thereby allow an increase in data rate without the need to increase the symbol transmission rate.